Process for making nano-sized and sub-micron-sized lithium-transition metal oxides

ABSTRACT

A process is provided for making nano-sized or sub-micron sized oxides of lithium and a transition metal. The desired size is generally in the range 10 nm to 1000 nm and preferably in the range 10 nm to 100 nm. The particles have a narrow size distribution. The process includes milling and re-firing under controlled conditions so that crystallites of the desired particle size are grown.

[0001] The present application claims priority to U.S. Ser. No.60/362,723 filed Mar. 8, 2002, the entire contents of which is herebyincorporated by reference.

[0002] The present invention relates to a process for producingnano-sized and sub-micron-sized lithium titanate, lithium manganate,lithium cobalt oxide and other oxides of lithium and transition metals.It covers parts of the process and the product of the process. Thestarting material is a coarse oxide with low surface area. The productmade according to the process of the present invention has a highsurface area and a narrow particle size distribution.

BACKGROUND OF THE INVENTION

[0003] Lithium-transition metal oxides are materials presently used orunder development for the electrodes of lithium ion batteries. Thetransition metals Co, Mn, Ni, Ti, and V have received particularattention for this application. Recently, it has become apparent that asmaller particle size and a narrower particle size distribution arebeneficial for producing electrodes, which retain their chargingcapacity at high charging and discharging rates.

[0004] A method to prepare lithium titanate from inorganic solutions orsuspensions is described in US Pat. Appln. Pub. 2003/0017104 A1, therelevant portions of which are incorporated herein by reference. Thatapplication describes a process to produce lithium titanatecrystallites. The process achieves good phase and size control in therange of 5 to 2000 nm. In general, the process includes providing asource of lithium titanate with a particle size smaller than the desiredparticle size and re-firing the lithium titanate under conditions toproduce a final lithium titanate having a desired particle size with anarrow size distribution and controlled surface area.

[0005] That application describes that a source of lithium titanate isfrom a process that includes forming a blend that comprises titanium andlithium. The blend is evaporated to form homogeneous particlescontaining a lithium salt and titanium dioxide. The evaporation isconducted at a temperature above the boiling point of the solution inthe blend but below the temperature where reaction of the lithium saltand the titanium dioxide occurs. The homogeneous particles are calcinedto form lithium titanate.

[0006] The lithium titanate is milled or crushed to a size smaller thanthe desired size of the final product. Finally, the milled lithiumtitanate is re-fired under conditions to produce lithium titanate havinga desired surface area and size distribution.

[0007] The blend of titanium and lithium can be provided from a varietyof suitable sources. For example, the blend of titanium and lithium isprovided as aqueous chloride solutions of titanium and lithium.Alternatively, the blend of titanium and lithium is provided as asuspension of amorphous titanium dioxide in a lithium solution. In thisinstance, the lithium solution can be formed from a source of lithiumselected from the group consisting of lithium chloride, lithium nitrate,lithium hydroxide, lithium sulfate, lithium oxide, lithium fluoride,lithium bromide, and mixtures thereof. In yet another alternative,lithium titanate, made by any known means and having a particle sizesmaller than the particle size of the desired product, can be used asthe source of lithium and titanium for the re-firing step, wherecrystals are grown to the desired size.

[0008] A method to prepare mixed metal oxides and metal oxide compoundsis also described in US Pat. Appln. Publication US 2002/0071806 A1 therelevant portions of which are incorporated herein by reference. Thismethod applies to mixed oxides of lithium and transition metals.Products made according to this patent application can be used asstarting materials for the process of the present invention.

[0009] Materials commercially available for the manufacture of batteryelectrodes generally have a wide particle size distribution and includelarge particles of several microns in size as well as very fine dust.Therefore, there is a need for materials having a narrow sizedistribution and having a controlled surface area for such applicationsas electrodes for batteries.

SUMMARY OF THE INVENTION

[0010] The present invention provides a process to producelithium-transition metal oxides in the range 10 to 1000 nm, andpreferably 10 to 100 nm with a narrow particle size distribution. Thephrase narrow particle size distribution means that the particle size ofthe lithium-transition metal oxide is within 10 nm to 1,000 nm with astandard deviation of no more than 20%.

[0011] In general, the process starts from a coarse oxide of lithium anda transition metal. The coarse oxide of lithium and a transition metalcan be provided by any suitable method including using commerciallyavailable coarse oxides. The process produces nano-sized crystalsthrough a combination of milling, dispersion and calcining (refiring)steps. The transition metal may be any metal commonly defined astransition metal, including but not limited to Ti, Co, Mn, V, Fe, andNi.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a flow sheet of the general aspect of the process.

[0013]FIG. 2 is a flow sheet of the process according to one embodimentof the present invention, where the starting material is lithiumtitanate spinel and the final product is a slurry containing nano-sizedparticles.

[0014]FIG. 3 is a flow sheet of the process according to the invention,where the starting material is lithium manganate LiMn₂O₄ or lithiumcobalt oxide LiCoO₂ and the final product is a nano-sized dispersedlithium manganate or lithium cobalt oxide powder.

[0015]FIG. 4 is a scanning electron micrograph of lithium titanatespinel of about 1 to 2 μm in size, serving as starting material for theprocess of the present invention.

[0016]FIG. 5 is a scanning electron micrograph showing lithium titanatespinel products of different particles sizes, produced following theprocess of the present invention.

[0017]FIG. 6 is a scanning electron micrograph showing commerciallithium manganate used as starting material for the process of thepresent invention.

[0018]FIG. 7 is a scanning electron micrograph showing lithium manganateproducts of different particle sizes produced following the process ofthe present invention.

DESCRIPTION OF THE INVENTION

[0019] According to the present invention, a process for makinglithium-transition-metal oxides is provided. In this process, a lithiumtransition-metal oxide is milled or crushed to a size smaller than thedesired size of the final product. The milled or crushed lithium-metaloxide is re-fired under controlled conditions to producelithium-transition-metal oxide (e.g., lithium titanate) having a desiredsurface area and size distribution. Further processing may includedispersion, remilling, slurrying and spray drying and the final productmay be a slurry, a spry dried powder consisting of agglomerates ofnano-particles, or a fully dispersed powder.

[0020] Turning now to FIG. 1, a flow sheet according to the generalprocess is shown. Coarse lithium-transition-metal oxide particles aremilled 40 to a desired median size. After milling, thelithium-transition-metal oxide (e.g., lithium titanate) is dried 45 andre-fired in a controlled temperature furnace 50 to produce particleshaving a desired size and size distribution.

[0021] Thereafter, the particles produced from the re-firing can bedispersed 60 or can be milled 90. If the particles are dispersed, theymay be further processed or may be left as is. Further processing mayinclude forming a lithium transition-metal oxide slurry 70, which can befurther processed by spray drying 80 to produce spray-dried powderagglomerates that consist of primary particles. The spray-dried powderagglomerates may be sold or may be further processed by milling 90 toproduce a fully dispersed powder.

[0022] The specific steps of the process will be explained in moredetail below.

[0023] Starting Material

[0024] The starting material of coarse particles can be made by anymethod. For example, commercially available coarselithium-transition-metal oxide particles can be used as the startingmaterial. Alternatively, as noted above, one suitable method isdescribed in US Pat. Appln. Publication 2003/0017104 A1, the relevantportions of which are incorporated herein by reference. While theprocess described in US Pat. Appln. Publication 2003/0017104 is directedto lithium titanate; it has now been found that the described processcan be used to make the lithium transition-metal oxides described in thepresent application. In addition, a related method is described in USPat. Appln. Publication 2002/0071806, the relevant portions of which areincorporated herein by reference.

[0025] According to those processes, a blend of a transition metal andlithium is provided by providing a source of lithium and a source of atransition metal. This blend may be referred to herein as thelithium-transition-metal blend or the transition-metal-lithium blend.

[0026] After the lithium-transition-metal blend is created, the blend isevaporated. The evaporation process is conducted above the boiling pointof the liquid in the blend and below the temperature where significantreaction of the lithium and the transition-metal compounds occurs orwhere there is significant crystallization of lithium-transition-metal.

[0027] The evaporation is conducted under conditions to achievesubstantially total evaporation and to form an intermediate. Inparticular, the evaporation is conducted at a temperature higher thanthe boiling point of the blend but lower than the temperature wheresignificant crystal growth of an oxide phase occurs. The evaporation maybe conducted at a temperature higher than the boiling point of the blendbut lower than the calcination temperature of the intermediate.

[0028] The term “substantially total evaporation” or “substantiallycomplete evaporation” refers to evaporation of greater than 85% of thefree water content, preferably greater than 90% of the free water andmore preferably greater than 95% of the free water present in the feedsolution. The term “free water” is understood and means water that isnot chemically bound and can be removed by heating at a temperaturebelow 150° C. After substantially total evaporation or substantiallycomplete evaporation, the intermediate product will have no visiblemoisture present.

[0029] The evaporation process is performed in a manner to control thephysical form of the product. Preferably, the evaporation process isaccomplished by spraying the blend while it is heated at a temperaturein the range from about 120° C. to about 350° C., and desirably in therange from about 200° C. to about 250° C. This process may be conductedin a spray dryer.

[0030] As noted in US Pat. Appln. Pub. 2003/0017104 A1, the evaporationprocess may be conducted in such a manner as to form a film of a mixtureof a lithium compound and an amorphous oxidized transition-metalcompound. In this regard, the evaporation process may be conducted insuch a way as to form a thin film of lithium salt on the preexistingparticles of amorphous oxidized transition-metal compound.

[0031] In both cases, through control of the operating parameters,including temperature and concentration of transition-metal and lithium,the characteristics of the solid intermediate product can be reliablycontrolled within a fairly narrow range. For example, the productresulting from injection in a spray dryer, will generally be composed ofhollow spheres or parts of spheres. The dimensions of the spheres mayvary from less than 0.1 μm to 100 μm or more in diameter and a shellthickness in the range from about 30 nanometer to about 1000 nanometeror more. The structure of the shell consists of an intimate mixture oftransition-metal and lithium compounds.

[0032] Evaporation by spraying also has the advantage of directprocessing of the solution so that a homogeneous intermediate product isformed and so that evaporation of water and acid is simultaneouslyaccomplished. Preferably, from about 90% to about 99% of any aqueousmaterial is evaporated.

[0033] The product resulting from the evaporation step is calcined at atemperature and for a length of time sufficient to convert the mixtureof transition-metal and lithium compounds to lithium transition metaloxide of the desired structure and particle size. Calcinationtemperatures can range between about 600° C. to 950° C. Desirably, thecalcination is conducted at temperatures ranging from about 700° C. toabout 900° C. The calcination time varies over a wide range, from about1 hour to as long as 36 hours. Desirably, the calcination time is in therange from about 6 hours to about 12 hours. Lower temperatures willrequire longer calcination times. The product of calcination shows astructure of individual units that can be broken up by milling intoparticles of the desired median size and size distribution.

[0034] During calcination, the lithium salt reacts with oxygen and waterin the furnace atmosphere to release, for example, HCl gas or nitrousand nitric oxides or other gases formed by decomposition of the saltpresent in the original solution. These gases may be recovered. Thecalcination conditions are chosen such that contact with oxygen issufficient to substantially convert the mixture to a lithiumtransition-metal oxide with low impurity level.

[0035] The product of calcination may contain traces of the originallithium salt used as feed. To remove the traces of salt, the particlesmay be subject to one or several wash cycles. In each cycle, theparticles are mixed with water and are separated by settling orfiltration. The washing step is particularly useful if the lithium saltused is lithium chloride.

[0036] Milling

[0037] The lithium-transition metal oxide is suspended in water andmilled in a horizontal or vertical pressure media mill to crush thecrystals to a size smaller than the size desired in the final product.

[0038] Drying

[0039] The wet-milled particles are dried by any known means. Forexample, wet-milled particles may be dried in a spray drier at atemperature from about 120° to about 350° C., desirably from about 200°to about 250° C. Drying may also be part of the re-firing process.

[0040] Re-Firing

[0041] After milling or drying, the product is re-fired in acontrolled-temperature furnace to make a product with a well-controlledspecific surface area, consisting of regular-shaped crystals with anarrow size distribution. The refiring temperature is chosen to achievethe desired particle size and surface area of the product. In general,the re-firing temperature is between about 250° and 900° C., and the BETsurface area of the re-fired product is in the range 5 to 100 m²/g, withthe higher re-firing temperature corresponding to the lower specificsurface area.

[0042] Dispersing

[0043] After the refiring step, the product may be dispersed 60 toseparate the agglomerates formed during re-firing into distinctnano-sized particles. This step is generally accomplished afterslurrying the product in water. Alternatively, the product of there-firing step may be milled 90, i.e., dry-milled, preferably in ajet-mill.

[0044] Further Processing

[0045] Depending on the destination of the final product, the productfrom the dispersing step can be kept as a slurry, or spray-dried, orspray-dried and jet-milled as indicated in FIG. 1.

[0046] The following examples illustrate, but do not limit, the presentinvention.

EXAMPLES Example I

[0047] Lithium titanate made by spray drying according to US Pat. Appln.Publication 2003/0017104 A1 and described above was further calcined inan oxidizing atmosphere at a temperature of 800° C. for 12 hours. Theproduct after calcination consisted of crystals of Li₄Ti₅O₁₂ of about400 to about 1000 nm in size. FIG. 4 is a scanning electron micrographof the product serving as starting material for the process of thepresent invention.

[0048] The product of the calcination step was further suspended inwater and milled with 0.4 to 0.6 mm zirconia grinding media for 8 hours.The BET surface area of this product was 135 m²/g. This product wasrefired at constant temperature for 3 hours. FIG. 5 shows electronmicrographs of the product obtained after refiring at 900°, 650°, 500°,and 400° C. respectively. The particle size was about 100 nm at 400° C.,and increased to about 200, 500 and 1000 nm respectively at the highertemperatures. The micrographs show well-formed crystals with a narrowsize distribution.

Example II

[0049] Commercial lithium manganate, of particle size and shape shown inFIG. 6, was slurried as a 40 weight % suspension in water and was milledin a Premier Mill bead mill for 20 h. The product of this operation wasdried, and then calcined in ceramic trays placed in a constanttemperature furnace for 3 hours. The results of calcinations atdifferent temperatures are given in the Table below: Calcinationtemperature Particle size (C.) (nm) uncalcined 15 400° 20 450° 30 500°50

[0050] Electron micrographs of each of the samples are shown in FIG. 7.All calcined crystals are well formed and show narrow sizedistributions.

[0051] While the invention has been described in conjunction withspecific embodiments, it is to be understood that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, this inventionis intended to embrace all such alternatives, modifications, andvariations that fall within the spirit and scope of the appended claims.

What is claimed:
 1. A process for producing a nano-sized or sub-micronoxide of lithium and a transition metal comprising: a. milling a coarselithium transition-metal oxide; and, b. re-firing the lithiumtransition-metal oxide
 2. The process of claim 1 wherein the lithiumtransition-metal oxide is tetra lithium titanate spinel
 3. The processof claim 1 wherein the lithium transition-metal oxide is LiCoO₂.
 4. Theprocess of claim 1 wherein the lithium transition-metal oxide isLiMn₂O₄.
 5. The process of claim 1 wherein the milling is accomplishedby wet-milling in a bead mill.
 6. The process of claim 1 furthercomprising: a. dispersing the product resulting from re-firing toliberate crystallites; and, b. slurrying the crystallites to form asuspension.
 7. The process of claim 1 further comprising milling theproduct resulting from re-firing to form a dispersed powder.
 8. Theprocess of claim 6 further comprising spray-drying the suspension. 9.The process of claim 8 further comprising milling the spray-driedproduct to form a dispersed powder.
 10. A lithium-transition metal oxidewith a particle size between 10 nm and 1000 nm and a standard deviationof no more than 20%.
 11. A lithium-transition metal oxide with aparticle size between 10 nm and 1000 nm and a standard deviation of nomore than 20% made according to the process of claim 1.